1. Field of the Invention
The present invention relates to positive temperature coefficient thermistor devices including positive temperature coefficient thermistor elements and metal bodies.
2. Description of the Related Art
Positive temperature coefficient thermistor devices including metal bodies defining radiators and positive temperature coefficient thermistor elements have been used for warm-air heaters and auxiliary heaters for air conditioners.
Japanese Examined Patent Application Publication No. 7-34390, for example, describes a positive temperature coefficient thermistor device including positive temperature coefficient thermistor elements interposed between two radiator plates that are resiliently supported by springs at both sides thereof. FIG. 1 shows the structure of the positive temperature coefficient thermistor device. The positive temperature coefficient thermistor device includes positive temperature coefficient thermistor elements 17 interposed between two radiator plates 11 and 13 that are resiliently supported by spring pins 19 at both sides thereof. The positive temperature coefficient thermistor elements 17 are insulated by a frame 15 and an insulating plate 18. In addition, electrodes on first surfaces of the positive temperature coefficient thermistor elements 17 are in contact with the radiator plate 11, and electrodes on the surfaces opposing the first surfaces are in contact with a terminal assembly 16.
Japanese Examined Patent Application Publication No. 7-34392 describes a positive temperature coefficient thermistor device including positive temperature coefficient thermistor elements biased into contact with the inner wall of a hollow metal body using a spring terminal. FIG. 2 shows the structure of the positive temperature coefficient thermistor device. Electrodes on first surfaces of positive temperature coefficient thermistor elements 27a and 27b are in contact with a metal body 25, and electrodes on first surfaces of positive temperature coefficient thermistor elements 28a and 28b are in contact with a metal body 26. The surfaces opposing the first surfaces are in contact with the spring terminal 29.
In the positive temperature coefficient thermistor device having the structure described in Japanese Examined Patent Application Publication No. 7-34390, both ends of the radiator plates are resiliently supported. Therefore, warpage of the radiator plates may cause insufficient contact of the positive temperature coefficient thermistor elements with the terminal assembly, and may cause electrode burn-out of the positive temperature coefficient thermistor elements. On the other hand, since the positive temperature coefficient thermistor device having the structure described in Japanese Examined Patent Application Publication No. 7-34392 includes the positive temperature coefficient thermistor elements, and the spring terminal inside the terminal assembly, and an insulating plate inside the hollow metal body, unwanted substances do not enter due to the hermetically sealed structure. Moreover, since the spring terminal directly pushes against the positive temperature coefficient thermistor elements, the components are reliably brought into contact with each other.
However, it is difficult to produce the positive temperature coefficient thermistor device having the structure described in Japanese Examined Patent Application Publication No. 7-34392 due to the complicated assembly of the positive temperature coefficient thermistor elements, and the spring terminal inside the hollow metal body.
Moreover, recent positive temperature coefficient thermistor devices, such as warm-air heaters and auxiliary heaters for automobiles, using positive temperature coefficient thermistor elements must have an output power of approximately 600 W. Heaters for automobiles, which use power sources of 12 volts DC, must pass currents of approximately 50 A through hollow metal bodies and spring terminals when the output power is approximately 600 W. However, with the positive temperature coefficient thermistor device having the structure described in Japanese Examined Patent Application Publication No. 7-34392, it is difficult to produce a spring terminal that is not burned out when a current of approximately 50 A passes through the terminal. That is, in order to obtain a spring terminal that can withstand high current, materials with low resistivity, for example, copper alloys such as phosphor bronze, must be used. However, when such copper alloys are used for the spring terminal having the structure described in Japanese Examined Patent Application Publication No. 7-34392, the terminal is easily deformed by the heat of the heater due to the low heat resistance of the copper alloys, which results in a reduction in the spring force. Consequently, the contact between the positive temperature coefficient thermistor elements and the hollow metal body is reduced, and as a result, the output power of the heater is reduced due to the reduced thermal conductivity. In contrast, when the spring terminal is made of stainless steel, which has high heat resistance, the electrical resistance of the spring terminal must be reduced by increasing the thickness of the material since the resistivity of the material is high. This leads to a problem that an optimum biasing force cannot be achieved due to the highly increased spring force.
Moreover, heaters for automobiles are often required to have a length of greater than about 200 mm. In this case, the spring terminal as described in Japanese Examined Patent Application Publication No. 7-34392 is bent while being inserted into the hollow metal body from an opening at an end of the hollow metal body, and thus is impractical. In addition, the spring terminal may damage the electrodes on the surfaces of the positive temperature coefficient thermistor elements while being inserted since the spring terminal is brought into direct contact with the positive temperature coefficient thermistor elements. This damage may lead to electrode burn-out.
Furthermore, the spring terminal is in contact with an insulator for insulating the spring terminal from the hollow metal body. The insulator may be damaged when a biasing force becomes concentrated in a portion of the insulator during insertion of the spring terminal. When an alumina substrate is used as the insulator, in particular, the substrate cannot function as the insulator when cracking occurs in the substrate.
In addition, silicone resin, which is a thermal conductor, is suitable for use as a soft insulator for such heaters. However, it is difficult to uniformly arrange silicone resin inside the hollow metal body described in Japanese Examined Patent Application Publication No. 7-34392.